R help - Jun 2008 - Problem with by(... , median)

Hello everyone,

I am new to R, I have been using SAS for a while. Not surprisingly, I 
find R much better in graphics, which is publication ready right away.

Recently, I have been trying to calculate some basic statistics using R. 
I have a dataset of multiple rows per subject. For example:

Subject	Date	Factor1	Factor2	Factor3
P1	0.5	1	1	3
P1	1	3	2	5
P1	2	3	5	NA
...
P2	0.5	1	6	4
P2	1	2	NA	7
P2	2	3	1	2
...

I wanted to get the 'by subject' statistics of the factors. I used the 
following code: a=by(dat1, dat1$Subject, mean, na.rm=TRUE) - this is for 
a mean.
However, when I try that with a 'median', I get an error message. It is 
strange to me, because with the 'summary' option, I get the median 
values for all factors separately, among other information. What I need 
is just a median, because I later need it for plotting.
I have read old posts on the archive, and I agree that the action of 
'median' is according to the documentation. However, I think this is a 
serious inconvenience, and it should be changed to work in the same way 
as mean, summary or sd options.

Nevertheless, accepting the reality, I have a question to all: how to 
obtain the by-subject medians of factors in such dataset?

Thanks in advance,

-- 
Michal J. Figurski
HUP, Pathology & Laboratory Medicine
Xenobiotics Toxicokinetics Research Laboratory
3400 Spruce St. 7 Maloney
Philadelphia, PA 19104

Try using aggregate:

aggregate(tb[-1], list(tb$Subject), FUN=median, na.rm = T)

On Tue, Jun 10, 2008 at 10:18 AM, Michal Figurski <
figurski@mail.med.upenn.edu> wrote:
> Hello everyone,
>
> I am new to R, I have been using SAS for a while. Not surprisingly, I find
> R much better in graphics, which is publication ready right away.
>
> Recently, I have been trying to calculate some basic statistics using R. I
> have a dataset of multiple rows per subject. For example:
>
> Subject Date    Factor1 Factor2 Factor3
> P1      0.5     1       1       3
> P1      1       3       2       5
> P1      2       3       5       NA
> ...
> P2      0.5     1       6       4
> P2      1       2       NA      7
> P2      2       3       1       2
> ...
>
> I wanted to get the 'by subject' statistics of the factors. I used
the
> following code: a=by(dat1, dat1$Subject, mean, na.rm=TRUE) - this is for a
> mean.
> However, when I try that with a 'median', I get an error message.
It is
> strange to me, because with the 'summary' option, I get the median
values
> for all factors separately, among other information. What I need is just a
> median, because I later need it for plotting.
> I have read old posts on the archive, and I agree that the action of
> 'median' is according to the documentation. However, I think this
is a
> serious inconvenience, and it should be changed to work in the same way as
> mean, summary or sd options.
>
> Nevertheless, accepting the reality, I have a question to all: how to
> obtain the by-subject medians of factors in such dataset?
>
> Thanks in advance,
>
> --
> Michal J. Figurski
> HUP, Pathology & Laboratory Medicine
> Xenobiotics Toxicokinetics Research Laboratory
> 3400 Spruce St. 7 Maloney
> Philadelphia, PA 19104
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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